1. Field of the Invention
The present invention provides a fiber optic cable for use in temperature measurement in locations inaccessible to humans, for example, the control line of an oil well. More specifically, it provides a double strand fiber optic cable incorporating a miniature bend at the end of the cable, connecting the double strands together.
2. Description of the Related Art
Temperature measurement in locations inaccessible to humans and equipment, for example, the hydraulic control line of an oil well, requires that some type of temperature sensing device be transported to the location where temperature measurement is desired, while overcoming the inaccessibility of the location to humans and equipment. The sensor may take the form of an optical fiber within a fiber optic cable, as explained in more detail below. The cable may be deployed by a procedure known as blowing, wherein high pressure air, water, or other medium is directed against the cable in the desired direction of travel, so that the frictional force imparted upon the cable forces the cable into the desired location. This procedure requires a cable having a sufficiently small diameter. If the distance involved is short, the cable may be pushed into position.
The use of fiber optic cables for temperature measurement in such locations has been proposed. The basic procedure is described in a paper presented by R. Normann, J. Wise, and J. Krumhansl, entitled “Points of Fibers Optic Cables For Permanent Geothermal Wellbore Deployment,” presented at the 26th Workshop on Geothermal Reservoir Engineering at Stanford University on Jan. 29-31, 2001. Once the cable is deployed in the appropriate location, a high powered pulsed laser is directed down the length of the fiber. The laser undergoes scattering throughout the fiber, the bulk of which results from unavoidable density fluctuations within the fiber. This scattered radiation is known as Rayleigh scattering, and occurs at the same wavelength as the incident radiation. Raman scattering makes a much smaller contribution to the overall scattering. Raman scattering is divided into Stokes scattering, occurring at a longer wavelength than the original pulse, and anti-Stokes scattering, occurring at a shorter wavelength. The Stokes and anti-Stokes photons exchange thermal vibrational energy with the atoms within the fiber, with the anti-Stokes photons absorbing thermal vibrational energy from these atoms. The vibrational energy of the atoms is a function of temperature, and therefore the ratio of Stokes and anti-Stokes signals is also a function of temperature.
It is possible to determine the temperature of the fiber at intervals along its entire length by recording and analyzing the Stokes and anti-Stokes signals as a function of time, and calculating the origination depth based on the time these signals are received. The distance to the originating point is calculated by the well-known formula of multiplying the travel time of the signal by the speed of light. It has been found that a ten nanosecond pulse length provides the ability to receive temperature readings about one meter apart.
Accordingly, there is a need for a fiber optic cable suitable for temperature measurement within the hydraulic control lines of oil wells that is also structured to be capable of installation in the desired location by blowing or other presently available procedures.